Process of producing stoichiometric uranium dioxide



F. L. TURBETT Jan. 31, 1967 PROCESS OF PRODUCING STOICHIOMETRIC URANIUMDIOXIDE Original Filed Oct. 27. 1959 United States Patent Thisapplication is a continuation of my copending application Serial No.848,954, filed October 27, 1959, for Process of Producing StoichiometricUranium Dioxide, now abandoned.

This invention relates to the production of uranium dioxide. Moreparticularly, this invention is concerned with a process of producing astoichiometric uranium dioxide having an extremely high density andpurity.

Uranium dioxide has. a theoretical density of 1097 grams per cubiccentimeter. It is important that the uranium dioxide in reactor fuelelements be as close to theoretical density, and stoichiometriccomposition, as possible because of space and heat conductivityproblems. The higher density fuel uses less space than the lower densitymaterial. This is a significant factor in reactor design. Also, asuranium dioxide approaches the stoichiometric form its heat conductivityincreases. This is important because it allows the heat of fissionwithin the fuel element to pass more rapidly to the surrounding medium,thereby increasing efficiency and preventing excessive buildup of heatwithin the fuel element.

Fuel elements for reactors are often produced by sintering ceramic gradeuranium dioxide previously pressed into the desired form. This processinvolves a large amount of waste because of warping of the fuel elementsand inability to produce sintered products of constant density.

Because of the difficulties encountered with sintering, swaging has beenemployed for producing reactor fuel elements. In this process, theuranium dioxide is placed into a pipe and subjected to immense pressuresthrough reduction of the pipe size. The density of the uranium dioxideafter swaging depends to a large extent upon the particle density beforeswaging so that high density material must be used. This is incontradistinction to the sintering process in which the final density isgoverned by the conditions of sintering, and is often raised above thatof the unsintered ceramic grade material. There is thus a need foruranium dioxide of high particle density.

According to the present invention there is provided a novel process ofproducing uranium dioxide of essentially stoichiometric density. Thisprocess comprises heating a uranium compound decomposable by heat intouranium dioxide until it melts, maintaining the uranium material at suchtemperature for a short time thereafter and rapidly cooling theresulting uranium dioxide in an atmosphere which is essentiallynonoxidizing to the hot fused uranium dioxide. The cooled fused uraniumdioxide then may be crushed, washed and dried.

Melting of the uranium feed material is advisably effected by anelectric are formed by a graphite electrode and the compound to befused.

The invention will now be described in conjunction with the attacheddrawing in which the figure is an electric are furnace used in thesubject invention. The furnace has a steel shell 1% containinginsulating refractory material 11 such as magnesite. A graphite plate 12diameter and 3" high) is placed on a layer of magnesite in the bottom ofthe furnace. A graphite cylinder (6%." diameter and 1" thick) iscentrally positioned with the bottom resting on the graphite plate 12. Aslightly internally Patented Jan. 31, 1%67 ice tapered graphite cylindershell 14 (4.25 top diameter; 4 bottom diameter and 1 thick at thebottom) is fitted into cylinder shell 13 so that its bottom also restson the graphite plate. The upper surface of the magnesite is coveredwith furnace brick 15 and high temperature alumina cement 16 used tohold the cylinder 13 and fire brick 15 in place.

The furnace is provided with a hood 24 for exhausting gaseous reactionproducts and recovering uranium therefrom. The hood has hollowcylindrical jacket 17 through which water enters at 18 and is removed at19. Conduit 2% is connected to a vacuum line which leads to a filter.This serves to remove fumes released in the process without loss ofuranium. Feed tube 21 is used to feed the uranium feed compound into thefurnace. Graphite rod 22 is an electrode, advisably the cathode, andgraphite rod 23 is the other electrode, advisably the anode.

In practicing this invention using an electric arc furnace, such asdescribed, a layer of the uranium feed material is first placed on thebottom of the furnace. The uranium compound that is fed to the hightemperature zone can be any compound of uranium that has sulhcientoxygen to form stoichiometric uranium dioxide. Examples of suchcompounds which can be used are ceramic grade uranium dioxide, U0 U0 U 0and ammonium diuranate. Alternately, any uranium compound can be used ifoxygen is supplied so as to allow the formation of U0 and the compounddecomposes at the temperature used. The material to be fused is conveyedthrough feed tube 21 by gravity and it falls through the water-cooledhood 24 into the areaof the electric arc. The are is first formedbetween the graphite electrode 22 and the inner wall 14 of the graphitefurnace. As the starting layer of the feed material is heated it becomesa conductor and the electrode then can be moved away from the wall sincethe arc is maintained between the electrode and the feed material. Atthis point the feed material is fed into the furnace at a rate so as tomelt it shortly after it reaches the arc. The tip of the electrode iskept as close to the accumulating mass of feed material as possible tokeep a steady arc and to utilize as much heat as possible. In somecases, depending upon the feed material, it is preferable to insert thetip of the electrode into the top portion of the accumulated feed.

Control of the arc is usually by sound and sighting through smokedglasses. The electrode is moved by the operator in order to melt anyfeed not previously melted when dropped into the are. An ammeter readingmay be used to aid in controlling the quality of the arc; however, sightand sound are also very important. Experience with the particularfurnace and feed material usually determines the optimum amperage flowfor a particular run. Excessive feed flow and improper electrodeposition are the conditions which most often cause a fluctuating arctemperature.

Alternating or direct current can be used. It is easier to maintain asteady temperature when direct current is used, and better control ofthe arc is obtained when the graphite electrode is the cathode. It isimportant that the feed material be fed at a constant rate to theheating zone so as not to unduly lower the temperature of such zone,since substantially complete melting of the material is essential.

In the operation of the furnace, the cross-sectional area of the furnaceincreases as a result of the furnace wall flaking off and the diameterof the electrode decreases because of the action of the are on it. Ithas been found desirable to increase the diameter of the electrode asthe furnace cross-sectional area increases so as to maintain a highenough temperature to substantially melt all of the uranium compound fedinto the arc. If, during the meltseolpeo L9 ing of any particular chargein the furnace, the tempera ture is allowed to fluctuate to a largeextent, a stoichiometric density material is not formed.

The described process functions as a purification operation because ithas been found that the temperature necessary to obtain a fused uraniumdioxide is high enough to volatilize many of the contaminants in thefuel.

After a predetermined amount of feed has been fused the are is continuedfor a short period of time in order to insure complete fusion. Theheated mass is then transferred, as quickly as possible to diminishcooling in air, to a nonoxidizing cooling medium. Advisably, the hotmass of uranium dioxide is removed from the electric furnace immediatelyafter the arc is terminated and rapidly cooled in water or in steam. Inthe case of water, it should be boiling so as to exclude most of theoxygen. A weak solution of ammonium hydroxide is useful in place ofwater since the ammonia is cracked at the surface of the molten mass andaids in reducing the formation of oxides higher than uranium dioxide.Any uranium carbides contained in the heated mass are converted tovolatile hydrocarbons, and mostly acetylene, and oxides and hydroxidesof uranium. In addition to water and steam, an inert atmospherecontaining water vapor or some other constituent that will convert theuranium carbides can be used.

An alternative method is to place the hot mass, immediately afterremoval from the high temperature zone, into an atmosphere of carbondioxide or some other inert atmosphere while such mass is being rapidlycooled. The uranium carbides are not completely converted into volatilehydrocarbons, and oxides and hydroxides of uranium in this way.Therefore, after the cooled mass is ground and washed, it is steamed toeliminate the uranium carbides. Without steaming, the conversionproducts that are not volatilized remain on the surface of the Uparticles. These products are not as dense as the fused U0 produced inthe furnace, and therefore lower the overall density of the finishedproduct unless steps are taken to remove them from the surface of thefused U0 The following table shows data to support this theory.

TABLE I A quantity of unfused feed material is removed from the furnacewith the fused material depending upon the size of the fused mass. Thisunfused material is on the surface of the fused mass and is returned tothe feeding hopper after it is separated in the washing step. While thematerial from the furnace is cooled in the water or in steam, adetectable odor is noticed. It is believed that any uranium carbide thatwas in the uranium compound comprising the feed, or was formed by thecombination of uranium with free carbon from the walls of the furnace orfrom the graphite electrode, is, at this point, converted tohydrocarbons, low oxides. or uranium hydroxides. When cooled, the massis easily crushed with slight pressure into crystals of stoichiometricuranium dioxide having the lower density uranium oxides and hydroxidecon taminants on the surface of said crystals. The contaminants are thenwashed out by use of a vibrating screen under water, a spray or othermeans. Any free carbon, unfused feed material, fines, and theaforementioned contaminants are then returned to the feed hopper.

The product is then dried. It is preferred to dry at between 60 C.70 C.If desired, the material may be ground to any desired fineness. Careshould be taken not to heat the uranium dioxide when grinding, otherwisethe material oxidizes to higher oxides of uranium. If this is notpossible, grinding should take place in an inert atmosphere.

The same procedure is followed when the molten mass is cooled in anonoxidizing atmosphere where a reactant is not present which canconvert the uranium carbides that were formed or were initially present,except that an additional step must be taken to convert such uraniumcarbides. This step consists of steaming the material after it has beenground to the desired particle size until there is no detectable odorcoming off the exhaust steam. This steaming step converts the uraniumcarbides to the aforementioned gaseous hydrocarbon, oxides andhydroxides of uranium; however, such oxides and hydroxides must then bewashed off the crystals of the stoichiometric U0 It can readily be beenthat the best method is to cool the hot mass coming from the furnace andconvert the carbides in one openation, such as cooling in theaforementioned boiling water or steam.

The following examples are presented to illustrate specific embodimentsof the invention.

Example 1 In an electric furnace consisting of a cylinder of graph ite,insulated so as to lose as little heat as possible to the surroundingatmosphere, a graphite rod, a water cooled hood and a feed tube throughthe hood, 109 pounds of ceramic grade uranium dioxide powder having abulk density of approximately 2 g./cc. was carefully fed at a uniformrate into the area of an electric are near the bottom of the furnace,for a period of four hours and minutes. The amperage was kept at about440 amperes and the voltage was kept at about volts. After a quantity ofabout five pounds of uranium feed compound was fed into the furnace, thefeeding was stopped and the are continued for about l2 minutes or untilthe operator was satisfied that substantially all the feed had beenmelted. Furnace charges of approximately five pounds each were melteduntil 109 pounds were consumed. Upon removal from the furnace the fusedoxide was rapidly cooled in boiling water at a depth about one half theheight of the fused mass. The mass was then gently tapped and it brokeinto uranium dioxide crystals. This material was then screened on a 20mesh vibrating screen under water. Any loose carbon from the furnacefloated to the surface. Undersized fused material and the unfused feedmaterial were dried and then returned to the feed hopper. This amountedto approximately 30.4% of the entire 109 pounds of feed, most of whichwas unfused U0 The finished product was stoichiometric and had a crystaldensity of 10.95 g./cc. or 99.82% of theoretical.

Example 2 Into the furnace described in Example 1 was fed 166 pounds ofceramic grade uranium dioxide having a bulk density of approximately 2g./cc. during a -minute in terval. The amperage was kept at about 330amperes and the voltage at about 55 volts. The procedure followed wasthe same as in Example 1. The finished product was stoichiometric andhad a density of 10.95 g./ cc. The feed loss was slightly in excess of1%. It is anticipated that in a larger operation this loss can bematerially reduced. Because it takes a certain amount of heat to raisethe temperature of the furnace it can be seen when comparing the outputof the furnace in each example that the capacity is greatly increased inproportion to the additional amount of power used.

Various changes and modifications of the invention can be made and, tothe extent that such variations incorporate the spirit of thisinvention, they are intended to be included within the scope of theappended claims.

What is claimed is:

1. A process for producing uranium dioxide comprising solidifying moltenuranium dioxide which contains uranium carbide as an impurity by coolingwith a fluid water containing medium which is nonoxidizing and inertthereto whereby the uranium dioxide is maintained in thatoxygen-containing state, and reacting the said uranium carbide impuritywith water to convert the uranium carbide to at least one substanceselected from the group consisting of volatile hydrocarbons, oxides ofuranium and hydroxides of uranium.

2. The process of claim 1 wherein the molten uranium dioxide issolidified by cooling with liquid water free of elemental oxygen and theuranium carbide impurity is reacted with the cooling water.

3. The process of claim 1 wherein the molten uranium dioxide issolidified by cooling with aqueous ammonium hydroxide and the uraniumcarbide impurity is reacted with water contained therein.

4. The process of claim 1 wherein the molten uranium dioxide issoldified by cooling in an atmosphere containing water in the form ofwater vapor.

5. The process of claim 1 wherein the uranium dioxide is washed afterreacting the uranium carbide impurity with the water to remove thereaction products thereof.

6. The process of claim 5 wherein the solidified uranium dioxide issubdivided to produce particles of stoichiornetric uranium dioxide whichhave the uranium carbide impurity on the surfaces thereof, the uraniumcarbide impurity is reacted with the water, and the particles of uraniumdioxide are washed to remove the resultant reaction prodnets.

7. The process of claim 6 wherein the uranium carbide is reacted withwater in the form of steam, and then the particles of uranium dioxideare washed with water to remove the reaction products.

8. The process of claim 7 wherein the molten uranium dioxide is cooledin an inert atmosphere.

9. The process of claim 8 wherein the inert atmosphere is gaseous carbondioxide.

10. The process for preparing uranium dioxide which comprises heating atleast one compound of the group consisting of U0 U0 U 0 U0 and uraniumdiuranate to an elevated temperature in an electric arc between a carbonelectrode and the uranium compound to form molten uranium dioxidecontaining uranium carbide as an impurity and cooling the molten uraniumdioxide in a fluid medium containing water which is inert to the moltenuranium dioxide to maintain the uranium dioxide in thatoxygen-containing state while converting the uranium carbide impurity toat least one substance selected from the group consisting of volatilehydrocarbons, oxides of uranium and hydroxides of uranium.

11. The process of claim 10 in which the fluid cooling medium is waterfree of elemental oxygen.

12. The process of claim 10 in which the fluid cooling medium is diluteaqueous ammonium hydroxide.

13. The process of claim 10 in which the fluid cooling medium includeswater vapor.

14. The process for preparing uranium dioxide which comprises heating atleast one compound of the group consisting of U0 U0 U 0 U0 and uraniumdiuranate to an elevated temperature in an electric are between a carbonelectrode and the uranium compound to form molten uranium dioxidecontaining uranium carbide as an impurity, cooling the molten uraniumdioxide in a water and elemental oxygen-free fluid medium which is inertto molten uranium dioxide, and steaming the cooled uranium dioxide toconvert the uranium carbide impurities to volatile hydrocarbons, oxidesof uranium and hydroxides of uranium.

References Cited by the Examiner UNITED STATES PATENTS 1,224,013 4/1917Parsons 23-145 1,274,794 8/1918 Shoeld 13-23 2,905,528 9/1959 Johnson23-145 2,906,598 9/1959 Googin 23-145 3,129,055 4/1964 Bel et a1. 23-355FOREIGN PATENTS 166,003 11/1955 Australia. 577,972 6/ 1959 Canada.

15,488 2/ 1897 Great Britain. 997,355 7/1965 Great Britain.

OTHER REFERENCES Ceramic Bulletin, vol. 36, No. 3, 1957, p. 114 only.

Chemical & Process Engineering, July 1960, Uranium Dioxide Fabrication,pp. 293 and 294.

Katz et al., The Chemistry of Uranium, National Nuclear Energy Series,Part 1, 1st Edition, pp. 260-262, 303, 304, 308-311 (1951).

Pedregal et al., Second UN. Conference on Peaceful Uses of AtomicEnergy, vol. 4, pp. -87.

CARL D. QUARFORTH, Primary Examiner.

REUBEN EPSTEIN, L. DEWAYNE RUTLEDGE,

Examiners. H. E. BEHREND, Assistant Examiner.

1. A PROCESS FOR PRODUCING URANIUM DIOXIDE COMPRISING SOLIDIFYING MOLTENURANIUM DIOXIDE WHICH CONTAINS URANIUM CARBIDE AS AN IMPURITY BY COOLINGWITH A FLUID WATER CONTAINING MEDIUM WHICH IS NONOXIDIZING AND INERTTHERETO WHEREBY THE URANIUM DIOXIDE IS MAINTAINED IN THATOXYGEN-CONTAINING STATE, AND REACTING THE SAID URANIUM CARBIDE IMPURITYWITH WATER TO CONVERT THE URANIUM CARBIDE TO AT LEAST ONE SUBSTANCESELECTED FROM THE GROUP CONSISTING OF VOLATILE HYDROCARBONS, OXIDES OFURANIUM AND HYDROXIDES OF URANIUM.